JP2002008022A - Method and system for compensating injection gradients in a capacitive sensing circuit array - Google Patents

Abstract

(57) [Problem] To provide a technique capable of accurately capturing a fingerprint image. The fingerprint sensor has an array of a plurality of capacitive pixel cells. Each of the capacitive pixel cells has a pair of metal plates and a reset transistor. The reset transistor has a gate, a source connected to one of the metal plates, and a drain connected to the other of the metal plates. A reset buffer generates a reset signal. A regenerator is associated with each reset transistor of each capacitive pixel cell. The regenerator regenerates the reset signal received from the reset buffer to remove the injection gradient across the array.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates generally to a method and system for capturing a fingerprint image, and more particularly to a method and system for compensating for an injection gradient in a capacitive fingerprint sensing circuit array. Things.

[0002]

2. Description of the Related Art Fingerprint recognition has been proposed for use in many security applications, such as controlling access to buildings, computers, and the like. The fingerprint recognition system can provide the user with no need to provide a device such as a keypad or card reader, and without having to memorize a password or other personal identification number or carry a card key. Can access facilities that are controlled.

[0003] One important element of a fingerprint detection system is the sensing device. One example of a sensing device is a TouchChip ™ silicon fingerprint sensor available from Estee Microelectronics, Inc. The touch chip uses an active pixel array based on a capacitive feedback sensing circuit. The array represents 3 pixels
It has a cell consisting of 60 rows and 256 columns.
Each pixel cell includes a high gain amplifier connected to two adjacent upper metal plates separated from the skin surface by a super-hard protective coating. The amplifier input is connected to one of the upper metal plates and the inverter output is connected to the other upper metal plate. The cell provides a charge integrator whose feedback capacitance is the effective capacitance between the two upper metal plates.

When a finger is placed on the sensor, the skin surface on the pixel cell acts as a third plate separated from the two adjacent plates by a dielectric layer of air. Since the fingerprint valleys are further away from the sensor surface than the fingerprint ridges, the pixel cells below the fingerprint valley are more likely to be located between their upper metal plate and the skin surface than the pixel cells below the fingerprint ridge. Has a greater distance. The thickness of the dielectric layer modulates the capacitive coupling between the top metal plates of the pixel cells, so that the top metal plate below the fingerprint valley presents a different effective capacitance than the top metal plate below the fingerprint ridge. I do.

[0005] Pixel cells operate in three phases. The first phase is reset, in which case the inputs and outputs of the charge integrator pixel cells are reset via CMOS reset transistors driven by reset signals. The second phase disconnects the output and input plates by asserting the reset signal driving the reset transistor to ground. By opening the reset transistor switch, channel charge is injected into both the input and output plates. During the third phase,
A fixed charge is applied to the charge integrator input, which causes the output voltage to swing in inverse proportion to the feedback capacitance, which is the effective capacitance between the upper metal plates. Because the distance between the skin and the pixel cell changes the effective feedback capacitance of the charge integrator, the output of the pixel cell below the fingerprint is different from the output of the pixel cell below the fingerprint valley.

In accordance with the concept of the present invention, when the reset transistor is active or on, there is a conduction path channel extending from the source to the drain of the reset transistor. As the gate voltage decreases and the reset transistor switches off, mobile carriers are drained from the channel through both the source and drain ends. The amount of channel charge injected into the input offsets the output of the charge integrator and corrects the image background. The amount of charge injection depends on several factors. These factors include the slope of the signal applied to the gate, the input / output capacitance ratio, and the dimensions of the reset transistor itself.

The reset signal is driven by a common buffer located at the top of the array. As the distance between the common buffer and the local pixel cell increases, the slope of the reset signal becomes lower due to the RC loading of the line. Changes in the slope of the reset signal over the length of the line will produce different charge injections as the distance from the output of the buffer increases. At the top of the array near the reset buffer, the amount of injected charge causes the pixel to reach its maximum saturated level, giving a very dark image. As one approaches the bottom of the array, the amount of injected charge is reduced, making the image brighter. Brighter images near the bottom of the array may make it more difficult to identify certain fingerprint features, which may lead to inaccurate fingerprint recognition. The injection gradient problem is particularly severe in large arrays where the distance from the reset buffer to the bottom of the array is significant.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and provides a technique capable of solving the above-mentioned disadvantages of the prior art and accurately recognizing a fingerprint. The purpose is to: It is another object of the present invention to provide a method and system for compensating for an injection gradient in a capacitive fingerprint sensing circuit array.

[0009]

SUMMARY OF THE INVENTION The present invention provides a capacitive fingerprint sensor and method for capturing a fingerprint image capable of compensating for an injection gradient. The fingerprint sensor of the present invention has an array of a plurality of capacitive pixel cells. Each of the capacitive pixel cells has a pair of metal plates, a charge integrator, and a reset transistor. The source of the reset transistor is connected to one of the metal plates, and the drain is connected to the other of the metal plates. The fingerprint sensor of the present invention has a plurality of reset signal regenerators, and there is at least one regenerator associated with each capacitive pixel cell.

[0010] In a preferred embodiment, the regenerator has an inverter. Each local inverter receives input from a global reset line driven by a common buffer and reproduces the slope of the reset signal that directly controls the CMOS switch. The fingerprint sensor of the present invention has a common reset buffer for generating a reset signal. The reset buffer has an output, which connects to the input of each local inverter via a common line. When the reset buffer generates a reset signal, each inverter reproduces the reset signal gradient at each reset transistor of each capacitive pixel cell, thereby making the reset signal gradient, and thus the injection, equal for all pixel cells. Let it. Thus, this compensates for the injection gradient along the columns of the array of capacitive pixel cells.

[0011]

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, a fingerprint detector according to the present invention is indicated generally by the reference numeral 11. The fingerprint detector 11 has a sensor array 13 and appropriate outputs indicated generally at 15. As explained in more detail below,
The sensor array has a rectangular array of a plurality of capacitive pixel cells arranged in rows and columns. In a preferred embodiment, these capacitive pixel cells have a pitch of about 50 microns, which is about 50 microns.
It corresponds to a resolution of 8 dpi. As shown in FIG. 1, the sensor array is dimensioned to capture a fingerprint image of a finger 17 placed thereon. The sensor array 13 is preferably formed on a single semiconductor chip.

Referring to FIG. 2, the configuration and operation of a capacitive pixel cell 19 according to the present invention is shown. The cell of the preferred embodiment of the present invention is a capacitive distance sensor (Capa).
Inventor Ta, filed February 13, 1997, entitled "Civive Distance Sensor".
rtagni, US patent application Ser. No. 08 / 799,548, the disclosure of which is incorporated herein by reference. Each cell 19 has a first conductor plate 21 and a second conductor plate 23 supported on a semiconductor substrate, the substrate preferably comprising a shallow epitaxial layer defining its upper surface area. It is a silicon substrate. The upper surface of the substrate has an insulating layer 25. The insulating layer 25 is preferably an oxide layer, which can be a thermally grown silicon dioxide layer. The conductor plates 21 and 23 are covered by a protective coating 27 of a hard material, which protects the cells 19 from moisture, contamination, wear,
And protect the cells 19 from electrostatic discharge.

Each cell 19 has a high gain inverting amplifier 29. The input of the inverting amplifier 29 is connected via an input capacitor 31 to a reference voltage source V _REF . Inverting amplifier 2
The output of No. 9 is connected to the output V _OUT . Conductor plate 2
1 and the input of the inverting amplifier 29, which is connected to the output of the inverting amplifier 29, are connected to a conductor plate 23, thereby forming a charge integrator, whose feedback capacitance is between the conductor plates 21 and 23. Is the effective capacity of

When a finger 33 is placed on the surface of the protective coating 27, the skin surface on each cell 19 is separated from the adjacent conductor plates 21 and 23 by a dielectric layer containing the protective coating 27 and air of variable thickness. Act as a spaced third capacitor plate. Valley of fingerprints 3
5 is further away from the conductor plates 21 and 23 than the fingerprint peak 37, so that the conductor plates 21 and 23
Cell 1 under fingerprint valley 35 between
9 presents a different effective capacity than the cell 19 below the fingerprint peak 37. The thickness of this dielectric layer modulates the capacitive coupling between plates 21 and 23 of each cell 19. Therefore, the cell 19 below the fingerprint valley 35 exhibits a different effective capacity than the cell 19 below the fingerprint ridge 37.

Cell 19 operates in three phases. During the first phase, the charge integrator is reset at switch 39 by shorting the input and output of inverting amplifier 29. Preferably, switch 39 is a reset transistor having a source connected to the input of inverting amplifier 29 and a drain connected to the output. Its input and output are shorted by applying a reset voltage to the gate of the reset transistor. The second phase is
Opening the switch 39 by applying ground to the gate of the reset transistor disconnects the output and input. By applying ground to the gate of the reset transistor, a phenomenon called injection occurs, in which charge is injected into both the input and output plates. During the third phase, a fixed charge is applied to the charge integrator input, which causes the output voltage to swing in inverse proportion to the feedback capacitance, which is the effective capacitance between conductor plates 21 and 23. For a fixed amount of input charge, the output of the inverting amplifier 19 is 2 depending on the charge injection and the effective feedback capacitance value.
Between the two limits. The first limit is the saturation voltage level when the effective feedback capacitance is very small. The second limit is the voltage near the logic threshold below the reset value when the effective feedback capacitance is large.

During the first phase, when the reset transistor of switch 39 is active or on, there is a conductive path channel extending from the source to the drain of the reset transistor. During the second phase, as the gate voltage on the reset transistor decreases, mobile carriers are ejected from the channel via both the source and drain ends. The percentage of the channel charge injected into the input relative to the total channel charge depends on several factors. These factors include the slope or slope of the signal applied to the gate, the input / output capacitance ratio, and the dimensions of the reset transistor itself. This charge injected into the input of the pixel charge integrator overlaps the input signal and modifies the output of the pixel cell.

Referring now to FIG. 3, one column of a prior art sensor array is shown. The reset signal is driven by a reset buffer 41 located at the top of the array. As the distance from the output of the reset buffer to the reset transistor 40 increases, the slope of the reset signal becomes lower due to the RC load on that line. Changes in the slope of the reset signal over the length of the line cause less charge injection as the distance from the output of the buffer increases. At the top of the array near the reset buffer, the amount of injected charge causes that pixel to reach its maximum saturation level, giving a very dark image. As it approaches the bottom of the array, the amount of charge injected decreases, making the image lighter.
Brighter images near the bottom of the array may make it more difficult to identify certain fingerprint features,
This may result in incorrect fingerprint recognition.

Referring now to FIG. 4, there is shown one column of a sensor array according to the present invention. This column of the present invention is generally similar to the column of FIG. 3, and therefore like elements have like reference numbers. However,
This column of the invention has means for regenerating the reset signal gradient at the reset transistor 40 of each cell 19. In the illustrated embodiment of the invention, the regeneration means includes an inverter 43 associated with each reset transistor 40. When the reset signal from the reset buffer 41 is applied to the input of the inverter 43, the inverted reset signal is applied to the gate of the reset transistor 40. The gradient of the inverted reset signal applied to each reset gate 40 is the same even if the gradient of the reset signal received by each inverter 43 is different. Therefore, the charge injected in each cell 19 is the same. Thus, the arrays of the present invention do not suffer from the injection gradient problem of the prior art.

As will be appreciated by those skilled in the art, inverter 43
Appropriate design steps must be taken to account for the inversion of the reset signal at. When the reset signal is asserted, it must cause the reset transistor 40 to conduct. One solution is to connect two inverters 43 in series to the gate of each reset transistor 40 if space is available. Another solution is to use a single inverter 43 with an inverted reset signal or P-channel reset transistor 40.

Although specific embodiments of the present invention have been described in detail above, the present invention should not be limited to these specific examples, but may be variously modified without departing from the technical scope of the present invention. Of course is possible.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram showing a fingerprint detection system according to the present invention.

FIG. 2 is a schematic diagram illustrating the physical structure and electrical operation of an individual capacitive pixel cell according to the present invention.

FIG. 3 is a schematic block diagram showing one column of an array of capacitive pixel cells according to the prior art.

FIG. 4 is a schematic block diagram illustrating one column of an array of capacitive pixel cells with injection gradient compensation according to the present invention.

[Explanation of symbols]

 Reference Signs List 11 fingerprint detector 13 sensor array 15 output 19 cell 21, 23 conductor plate 25 insulating layer 27 protective coating 29 high gain inverting amplifier

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Marco Sabatini United States of America, California 94707, Kensington, Edgecroft Way 67 (72) Inventor Frederick Reynal United States of America, California 94707, Berkeley, Yosemite Road 1826 (72) Inventor Giovanni Gozzini United States, California 94709, Berkeley, Martin Luther King Jr. Way 1508 F-term (reference) 4C038 FF01 FG00 5B047 AA25 5J050 AA05 AA37 BB23 CC06 CC19 DD08 EE22

Claims (12)

[Claims] 1. A fingerprint sensor, comprising: a capacitive pixel cell comprising a pair of metal plates and a reset transistor, wherein the reset transistor is connected to a gate and one of the metal plates. A capacitive pixel cell having a source connected to the other of the metal plates and a drain connected to the other of the metal plates; and an inverter having an input and an output, wherein the output is the gate of the reset transistor. And a reset buffer having an output connected to an input of the inverter via a common line. 2. The high gain inverter according to claim 1, comprising an input and an output, wherein the input of the high gain inverter is connected to one of the metal plates and the high gain inverter. A high gain inverter connected to the other of said metal plates, and a charge buffer having an output operatively connected to an input of said high gain inverter. Fingerprint sensor. 3. The fingerprint sensor according to claim 2, further comprising an input capacitor connected between an output of the charge buffer and an input of the high gain inverter. 4. A method for capturing a fingerprint image, comprising: providing a plurality of capacitive pixel cells, each of said capacitive pixel cells having a pair of metal plates and a reset transistor, wherein said reset transistor has a gate. And a source connected to one of the metal plates, and a drain connected to the other of the metal plates, the reset signal generating a reset signal to the plurality of pixel cells. Regenerating said reset signal to reset a reset transistor of each of said pixel cells. 5. A fingerprint sensor, comprising: a plurality of capacitive pixel cells, each of said capacitive pixel cells comprising a pair of metal plates and a reset transistor, wherein said reset transistor has a gate and said reset transistor. A plurality of capacitive pixel cells comprising a source connected to one of the metal plates and a drain connected to the other of the metal plates, connected to a gate of each reset transistor; And a reset buffer having an output connected to each inverter. 6. The high gain inverter of claim 5, wherein each capacitive pixel cell has an input and an output, wherein the input of the high gain inverter connects to one of the metal plates. A high-gain inverter having an output connected to the other of the metal plates and an output operatively connected to an input of the high-gain inverter. A fingerprint sensor characterized in that: 7. The method of claim 6, wherein each capacitive pixel cell further comprises an input capacitor connected between an output of the charge buffer and an input of the high gain inverter. And fingerprint sensor. 8. A fingerprint sensor, comprising: a plurality of capacitive pixel cells, wherein each of the capacitive pixel cells includes a pair of metal plates and a reset transistor, wherein the reset transistor has a gate and the reset transistor. A plurality of capacitive pixel cells comprising a source connected to one of the metal plates and a drain connected to the other of the metal plates; means for generating a reset signal; Means for regenerating said reset signal in each of said pixel cells. 9. The method according to claim 8, wherein the means for regenerating the reset signal includes a plurality of inverters,
Each of the inverters has an input and an output,
A fingerprint sensor, wherein there is at least one inverter having an output connected to the gate of each reset transistor. 10. The apparatus of claim 9, wherein said means for generating a reset signal comprises a reset buffer having an output connected to an input of an inverter of each of said capacitive pixel cells. Features a fingerprint sensor. 11. The high gain inverter of claim 8, wherein each capacitive pixel cell has an input and an output, wherein an input of the high gain inverter connects to one of the metal plates. A high-gain inverter having an output connected to the other of the metal plates and an output operatively connected to an input of the high-gain inverter. A fingerprint sensor characterized in that: 12. The method of claim 11, wherein each capacitive pixel cell further comprises an input capacitor connected between an output of the charge buffer and an input of the high gain inverter. Fingerprint sensor.